a) Field of the Invention
The present invention relates to a magneto-resistance (MR) reading head used for hard disks or the like, and more particularly to an MR reading head capable of reading data recorded on a narrow track at a high density, by suppressing the generation of a side-lobe which shows an off-track characteristic specific to an MR element, while preventing a sensitivity from being lowered.
b) Description of the Related Art
An MR head is a magnetic head dedicated to reproducing (reading) information recorded on a magnetic recording medium with an MR element by detecting a magnetic field generated from storage magnetic poles in the recording medium. As compared to an induction type magnetic head, an MR head has an advantage in improving a track density and a linear density of recording. Therefore, an induction-MR type composite magnetic head for hard disks is made of a combination of an MR head and, for example, a recording (writing) induction type head.
A conventional induction-MR type composite magnetic head for hard disks is shown in FIGS. 7A and 7B. FIG. 7A is a cross sectional view showing the side elevation of the composite magnetic head, and FIG. 7B is a perspective view thereof as seen from the recording medium plane side. The induction-MR type composite magnetic head 10 is constituted by a lamination of an MR type magnetic head (MR head) 12 and an induction type magnetic head 14 both being formed on a rear end portion of a substrate 16 having a slider surface 24. Both the heads 12 and 14 are manufactured by adopting thin film forming technique.
A lower shield layer 18 of the MR type magnetic head 12 is formed on the slider substrate 16 at its read end portion. The rear end portion of the slider substrate 16 is flat and the lower shield layer 18 is of a flat sheet structure. A lower gap layer 20 of non-magnetic material is laminated upon the lower shield layer 18. A sensor unit 28 is formed on the lower gap layer 20 with its end surface being directed toward a counter surface (slider surface) 24 of a recording medium. The sensor unit 28 is made of an MR film 46, a magnetic spacer layer 48, and a soft adjacent layer (SAL) 50 deposited in this order on the lower gap layer 20 and made flat. As shown in FIG. 7B, on the opposite end portions of the sensor unit 28, leads (lead conductors) 30 and 31 are laminated to establish electrical contacts to the sensor unit. The region of the MR film 46 not overlapped with the leads 30 and 31 is an active region whose resistance is changed by a magnetic field. The region of the MR film 46 overlapped with the leads 30 and 31 is an inactive region whose resistance change cannot be detected because of the high conductivity leads 30 and 31. An upper gap layer 32 of insulating material such as alumina is formed on the sensor unit 28 and leads 30 and 31. An upper shield layer 34 is formed on the upper gap layer 32. The lower and upper shield layers 18 and 34 are formed by depositing soft magnetic material having a high permeability through sputtering, vapor deposition, plating, or the like.
The induction type magnetic head 14 uses the upper shield layer 34 of the MR type magnetic head 12 as its lower core. Sequentially deposited on the upper shield layer 34 are a gap layer 36, a coil 37 buried in an insulating layer 38, an upper core 40, and a protection film 42. The gap layer 36 separates the upper and lower cores 40 and 34 at the lower end as shown in FIG. 7A to form a magnetic gap.
In recording data with the induction-MR type composite magnetic head 10 shown in FIGS. 7A and 7B, a recording current is flowed through the coil 37 of the induction type magnetic head 14 to generate a recording (leakage) magnetic field in the magnetic gap between the upper and lower cores 40 and 34 and record data in a recording medium by this magnetic field. In reproducing data, a predetermined current is flowed through the MR film 46 via the leads 30 and 31 of the MR type magnetic head 12 to magnetically saturate the SAL film 50 and apply a transverse bias magnetic field to the MR film 46 by this saturated magnetic field. A current flowing through the MR film 46 via the leads 30 and 31 is a sense current by which a voltage drop in the MR film 46 generated by a change in an applied magnetic field is detected. As the head 12 traces a track of the recording medium, a voltage across the MR film 46 is modulated by magnetization information on the track so that the recorded data can be detected.
The off-track characteristic of a magnetic head will be explained. FIG. 8A illustrates a read/write operation by a general induction type magnetic head. In a write operation, a signal is written on a track of a recording medium (hard disk) in a pole width Tw of the poles of the upper and lower cores 40 and 34. This written magnetization information is read by the induction type magnetic head. In this case, a maximum reproduction sensitivity is obtained when the poles reach the position just above the track width Tw. As the poles shift in a direction (track width direction) perpendicular to the motion direction of the recording medium, the reproduced output lowers in correspondence with a shift amount of the poles from the track width Tw. FIG. 8B illustrates the off-track characteristic O.sub.I. The abscissa represents a lateral shift from the track center, and the ordinate represents a reproduced output voltage in an absolute value. As shown in FIG. 8A, the lateral right shift of the poles is represented by a plus (+) sign, and the lateral left shift is represented by a minus (-) sign.
The off-track characteristic O.sub.I of a general induction type magnetic head shows a maximum sensitivity Vmax when the center of a track width of magnetization information written on a recording medium becomes coincident with the center of the pole width Tw. The reproduction sensitivity Vmax gradually lowers symmetrically with the right and left shifts of the poles, as the poles shift in the track width direction. A reproduction output of the induction magnetic head depends on a speed of the recording medium relative to the head, and is zero when the relative speed is zero.
For reproduction with the MR-induction type composite magnetic head, the MR type magnetic head is used. Since the MR type magnetic head detects a change in magnetic flux density as a change in resistance, it can detect magnetization information independently from a relative speed between the head and magnetic recording medium. FIG. 9 shows an example of the off-track characteristic of an MR type magnetic head. As shown, the off-track characteristic O.sub.MR is asymmetric with the track center and has a small bump called a side-lobe. The reason why this side-lobe is formed will be described with reference to FIGS. 10A to 10EA.
FIGS. 10A to 10E show relative positions of a track 52 written on a recording medium and a MR type magnetic head such as shown in FIGS. 7A and 7B. FIGS. 10AA to 10EA illustrate mechanisms of generating reproduction outputs by the MR type magnetic head at corresponding positions shown in FIGS. 10A to 10E. The relative positions of FIGS. 10A to 10E are shown as cross sections perpendicular to a plane of a recording medium.
An MR film 46 has an easy axis of magnetization in a horizontal direction or longitudinal direction as seen in FIGS. 10A to 10E. Of the MR film 46, the regions (inactive regions) overlapped with the leads 30 and 31 are called lead regions 46a and 46b, and the region between these lead regions is called an active region 46c. As magnetization in the active region 46c changes, a reproduction output changes. The width of the active region 46c is equal to the width of a track 52. It is assumed here that a sense current i flows through the MR film 46 from left to right and a magnetic field is generated upward in the MR film 46. It is also assumed that a magnetization vector of the track is upward.
Current flows through the MR film 46 to magnetically saturate the SAL bias film 50 (FIG. 7B) and this saturated magnetic field applies an upward bias magnetic field to the active region 46c. In order to obtain a maximum sensitivity, the magnetization direction is set about 45 degrees relative to the plane of a magnetic recording medium. Solid lines in the MR film 46 indicate magnetization in the MR film 46 without external magnetization, and broken lines indicate magnetization rotation in the MR film 46 caused by a magnetic field from the track 52.
In FIGS. 10AA to 10EA, a magnetization vector in the MR active region generated by a static bias is indicated by M1, a magnetization vector generated during a reproduction operation is indicated by M2, and the resultant vector is indicated by M3. Namely, the magnetization vector changes from M1 to M3 during the reproduction operation.
The plus (+) sign in the abscissa in FIG. 9 corresponds to a track left shift relative to the active region 46c, and the minus (-) sign corresponds to a track right shift. FIGS. 10A to 10E stand for two minus values, 0, and two plus values.
As shown in FIG. 10A, when the track 52 is at the minus position and under the lead region 46a, magnetization of the lead region 46a is influenced by a magnetic field of the track 52. Therefore, magnetization in the active region 46c receives a clockwise vector as shown in FIG. 10AA. The influence of this clockwise vector becomes larger, the higher the uniaxial anisotropy of the MR film 46.
As shown in FIG. 10B, when the track 52 moves right and rides over the lead region 46a and active region 46c, the magnetic field of the track 52 directly influences the magnetization of the active region 46c and further gives a clockwise vector to the active region 46c as shown in FIG. 10BA. Therefore, a reproduction output further increases.
As shown in FIG. 10C, as the track 52 further moves right to the position coincident with the active region 46c, the direct influence of the track 52 upon the active region 46c becomes maximum. At this time, the influence of the clockwise magnetization vector is strongest and the output is maximum.
As shown in FIG. 10D, as the track 52 further moves right from the track center and rides over the active region 46c and lead region 46b, the magnetic field of the track 52 influences the lead region 46b to rotate the magnetization of the active region in a counter-clockwise direction. Therefore, as shown in FIG. 10DA, the function of clockwise rotation of the magnetization in the active region is cancelled and the reproduction output rapidly lowers. The reproduction output becomes zero when the clockwise magnetization vector in the active region 46c directly influenced by the magnetic field of the track 52 balances just with the counter-clockwise magnetization vector in the active region 46c influenced by the magnetization in the lead region 46b caused by the magnetic field of the track 52.
As shown in FIG. 10E, as the track 52 further moves right and reaches under the right lead region 46c, The magnetic field of the track 52 influences the magnetization in the lead region 46b, and the magnetization in the active region 46c is rotated in the counter-clockwise direction. Therefore, as shown in FIG. 10EA, the magnetization vector is rotated in the counter-clockwise direction and a reproduction output of an opposite phase is generated. This opposite phase output gradually reduces as the track 52 further moves right away from the lead region 46b.
From the above reason, the off-track characteristic becomes asymmetrical as shown in FIG. 9. The narrower the width (track width) of the active region 46c is set, the larger the side lobe SL. If the direction of current is reversed, this relation is also inverted and the side-lobe SL appears on the left side. Such a side-lobe becomes a vital obstacle against a reliable tracking servo and makes it impossible to perform a servo control of a narrow track. High density recording is therefore hindered.
As a method of reducing a side-lobe, it can be considered that magnetization (in the same direction of an anisotropic magnetic field in the MR film 46) of uniaxial anisotropy bias magnet films on the right and left sides of the MR film 46 is made stronger. However, in this case, in order to maintain the direction of magnetization in the MR film 46 at 45.degree., a bias magnetic field of the SAL bias film 50 is required to be made stronger. Therefore, a change in the direction of magnetization in the MR film 46 with the magnetic field from a recording medium becomes small and the reproduction sensitivity lowers. This is conspicuous particularly for narrow tracks.